The present invention relates to a magnetic circuit for a dynamic speaker. More particularly, the invention relates to an external magnet type magnetic circuit including an annularly- shaped magnet, a pole yoke having a back plate coupled to one surface of the magnet and a center pole extending from the central portion of the back plate and passing through a hole in the annularly-shaped magnet, and an annularly-shaped top plate coupled to the other surface of the magnet, in which an air gap is provided between the center pole and the top plate.
Prior art external magnet type magnetic circuits are shown in FIGS. 1A and 1B. In the magnetic circuit shown in FIG. 1A, the lower surface of a back plate 2a of a pole yoke 2 is flat while a stepped portion 2a' is formed in the upper surface of the back plate 2a to determine the position of the magnet 1. An air gap is provided between a top plate 3 coupled to the upper surface of the magnet 1 and a center pole 2b of the pole yoke 2.
The structure of this magnetic circuit is disadvantageous in that the thickness of the magnet must be increased as the length of the air gap is increased and hence as the circuit is made more effective. In order to resolve such a disadvantage, a magnetic circuit as shown in FIG. 1B has been proposed in which the back plate 2a protrudes downwardly at the base portion of the center pole 2b and a groove 2c is formed in the back plate 2a along the circumferential direction of the center pole 2b. According to the magnetic circuit thus constructed, the length of the air gap 4 in the moving direction of the voice coil (not shown) is advantageously increased without increasing the thickness of the magnet 1. Denoted by 2a" is a protruding portion provided to facilitate the positioning of the magnet 1.
On the other hand, the magnetic circuit shown in FIG. 1B is disadvantageous due to leakage of the magnetic flux. Specifically, there exists leakage magnetic flux extending from the lower surface of the top plate 3 to the surface of the protruding portion 2a" or to the pole yoke 2b.
Recently, a strontium ferrite magnet has been employed in this type of magnetic circuit, which provides a stronger magnetic flux in comparison with a conventionally used barium ferrite magnet. If such a strontium ferrite magnet is used in the magnetic circuit in FIG. 1B, the thickness of the magnet can further be decreased while maintaining the strength of the magnetic field in the air gap substantially constant. However, leakage of the magnetic flux then becomes significant. In addition, there is a further disadvantage in the structure of FIG. 1B in that the outer diameter of the magnet must be increased because the groove 2c and the protruding portion 2a" are formed inside the inner diameter of the magnet 1. Although the recent fabrication techniques for the magnet do not necessarily require such a protruding portion 2a", the same disadvantage still remains due to the presence of the groove 2c. To shorten the outer diameter of the magnet thus minimizing the magnet volume, the width of the groove 2c must be made shorter than the depth of the groove 2c. In the press-molding of the pole yoke 2, the groove 2c is also concurrently molded. If the mold die used is formed with a corresponding groove 2c, the die portion used to mold the groove 2c becomes thin and is hence liable to be damaged. For this reason, a cutting machine is practically used to form the groove 2c. The use of the cutting machine, however, increases the manufacturing cost and number of manufacturing steps. Further, the cut-out portion of the material is wasted. Consequently, the magnetic circuit thus manufactured is expensive.